The present invention relates to a protection switching method and apparatus for a PON (Passive Optical Network) system in which a plurality of ONUs (Optical Network Units) are star-connected to an OLT (Optical Line Terminal) through a photocoupler.
FIG. 24 shows the basic arrangement of a PON system.
As shown in FIG. 24, in an OLT 106, a transmission/reception section 101 is connected to a port of a switch (SW) 104, and the transmission/reception section 101 is connected to transmission/reception sections 103-1 to 103-n of a plurality of ONUs 107-1 to 107-n through a photocoupler 102 and optical fibers 112-1 to 112-n. The ONUs 107-1 to 107-n are star-connected to the single OLT 106.
The transmission/reception sections 103-1 to 103-n of the ONUs 107-1 to 107-n are respectively connected to subscriber terminals 109-1 to 109-n. When the transmission/reception section 101 of the OLT 106 is to communicate with one of the subscriber terminals 109-1 to 109-n, a control section 110 of the OLT 106 controls switching operation of the switch 104. With this operation, identical signals are distributed from the photocoupler 102 to the ONUs 107-1 to 107-n through the optical fibers 112-1 to 112-n, and one of the ONUs 107-1 to 107-n extracts the signal upon determining that the signal is self-addressed.
Assume that the ONU 107-1 determines that the signal is self-addressed. In this case, when a virtual path is established between the subscriber terminal 109-1 and the transmission/reception section 101 of the OLT 106, the transmission/reception section 101 of the OLT 106 can communicate with the subscriber terminal 109-1. That is, the transmission/reception section 101 of the OLT 106 can communicate with the subscriber terminal 109-1 through the photocoupler 102, optical fibers 112-1 to 112-n, and the transmission/reception section 103-1 of the ONU 107-1.
FIG. 25 shows a dual arrangement designed to ensure the reliability of a PON system having such an arrangement. In the dual arrangement shown in FIG. 25, an OLT 106 has sections of two systems, i.e., a 0-system transmission/reception section 101a and 1-system transmission/reception section 101b. Likewise, ONUs 107-1 to 107-n respectively have sections of two systems, i.e., 0-system transmission/reception sections 103-1a to 103-na and 1-system transmission/reception sections 103-1b to 103-nb. In order to implement two systems in this manner, the OLT 106 has a selector (SEL) 105 in addition to a switch 104 and control section 110.
The SEL 105 selectively switches between the 0-system transmission/reception section 101a and 1-system transmission/reception section 101b to connect the 0-system transmission/reception section 101a to the 0-system transmission/reception sections 103-1a to 103-na of the ONUs 107-1 to 107-n through a photocoupler 102a and optical fibers 112-1a to 112-na. In addition, the 1-system transmission/reception section 101b of the OLT 106 is connected to the 1-system transmission/reception sections 103-1b to 103-nb of the ONUs 107-1 to 107-n through optical fibers 112-1b to 112-nb. 
The 0-system transmission/reception sections 103-1a to 103-na and 1-system transmission/reception sections 103-1b to 103-nb of the ONUs 107-1 to 107-n are respectively selected by SELs 108-1 to 108-n of the ONUs 107-1 to 107-n in accordance with selection of 0-system or 1-system by the SEL 105 of the OLT 106. The selected 0-system transmission/reception sections 103-1a to 103-na or 1-system transmission/reception sections 103-1b to 103-nb are respectively connected to subscriber terminals 109-1 to 109-n. 
Assume that the PON system is operating with the 0-system transmission/reception section 101a and 0-system transmission/reception sections 103-1a to 103-na belonging to an active system, and the 1-system transmission/reception section 101b and 1-system transmission/reception sections 103-1b to 103-nb belonging to a standby system.
Note that the terms “0-system” and “1-system” are added to physically identify the respective sections. However, the 0-system transmission/reception section 101a and 0-system transmission/reception sections 103-1a to 103-na do not always belong to the active system, and the 1-system transmission/reception section 101b and 1-system transmission/reception sections 103-1b to 103-nb do not always belong to the standby system. That is, the 0-system and 1-system are irrelevant to the active and standby systems. The active system is a currently used system, and the standby system is a system that is used upon switching from the active system.
Assume that the 0-system is an active system in the following description. Referring to FIG. 25, the 0-system transmission/reception section 101a of the OLT 106 is now capable of communicating with the subscriber terminals 109-1 to 109-n through the photocoupler 102a, the 0-system transmission/reception sections 103-1a to 103-na and SELs 108-1 to 108-n of the ONUs 107-1 to 107-n. Assume that a virtual path is established between the subscriber terminal 109-1 and the 0-system transmission/reception section 101a of the OLT 106 through the photocoupler 102a and transmission/reception section 103-1a and SEL 108-1 of the ONU 107-1, and the 0-system transmission/reception section 101a is now communicating with the subscriber terminal 109-1.
When an abnormality occurs in the virtual path between the subscriber terminal 109-1 and the 0-system transmission/reception section 101a of the OLT 106 during this communication owing to some cause, no data is transmitted from the subscriber terminal 109-1 to the 0-system transmission/reception section 101a of the OLT 106. As a result, the 0-system transmission/reception section 101a detects the occurrence of the abnormality in the virtual path, and sends a warning signal to the control section 110.
Upon reception of the warning signal, the control section 110 outputs a switching instruction to the SEL 105 to switch from the 0-system transmission/reception section 101a to the 1-system transmission/reception section 101b. With this operation, all the virtual paths between the OLT 106 and subscriber terminals 109-1 to 109-n are switched to the 1-system at once. That is, virtual paths are established between the 1-system transmission/reception section 101b of the OLT 106 and the subscriber terminals 109-1 to 109-n through the photocoupler 102b, optical fibers 112-1b to 112-nb, and the 1-system transmission/reception sections 103-1b to 103-nb and SELs 108-1 to 108-n of the ONUs 107-1 to 107-n. 
With this operation, the communication between the OLT 106 and the subscriber terminal 109-1, which has been interrupted due to the occurrence of the abnormality, is resumed upon switching to the virtual path constituted by the 1-system transmission/reception section 101b, photocoupler 102b, optical fiber 112-1b, and 1-system transmission/reception section 103-1b and SEL 108-1 of the ONU 107-1.
FIG. 26 shows another example of the dual arrangement of a conventional OPN system.
In the arrangement shown in FIG. 26, control sections 111-1 to 111-n are added to the ONUs 107-1 to 107-n in FIG. 25. These control sections 111-1 to 111-n control SELs 108-1 to 108-n to switch (select) between 0-system transmission/reception sections 103-1a to 103-na and 1-system transmission/reception sections 103-1b to 103-nb. Since the arrangement of the remaining portion is the same as that in FIG. 25, the same reference numerals as in FIG. 25 denote the same parts in FIG. 26, and a description thereof will be omitted.
Assume that as in the case shown in FIG. 25, a fault has occurred in one of the following components of the 0-system: a transmission/reception section 101a, photocoupler 102a, optical fibers 112-1a to 112-na, and the transmission/reception sections 103-1a to 103-na of ONUs 107-1 to 107-n, while the PON system is operating with the 0-system serving as an active system, and the 1-system serving as a standby system.
The 0-system transmission/reception section 101a always monitors signals between the OLT 106 and ONUs 107-1 to 107-n, and notifies a control section 110 of the OLT 106 of an abnormality upon detecting a signal abnormality. Upon reception of the abnormality notification, the control section 110 outputs a switching command to an SEL 105 of the OLT 106 to switch the transmission path from the 0-system to the 1-system. As a consequence, connection between the OLT 106 and the ONUs 107-1 to 107-n is restored by using the 1-system optical transmission path.
Upon outputting the switching command to the SEL 105, the control section 110 outputs switching commands to the ONUs 107-1 to 107-n through the 1-system connection, i.e., the SEL 105-1, 1-system transmission/reception section 101b, photocoupler 102b, and the 1-system transmission/reception sections 103-1b to 103-nb of the ONUs 107-1 to 107-n. With this operation, the control sections 111-1 to 111-n of the ONUs 107-1 to 107-n control switching operation of the SELs 108-1 to 108-n to restore the transmission paths to subscriber terminals 109-1 to 109-n. 
In each of the dual arrangements of the conventional PON systems shown in FIGS. 25 and 26, however, even if a fault occurs in only the transmission/reception 103-1a of the 0-system ONU 107-1, which is part of the PON system, the overall PON system must be simultaneously switched from the 0-system to the 1-system in order to restore a path under communication. That is, switching is performed even for the ONUs 107-2 to 107-n that are operating normally. As a result, the communication quality deteriorates due to short breaks and the like caused in this operation.
In each of the arrangements of the conventional PON systems shown in FIGS. 25 and 26, the active and standby systems are physically discriminated from each other, and the standby system cannot be used until it is selected by the SEL 105. In addition, the active system is switched to the standby system by only interchanging the physical transmission paths, and only the same connection as the preceding connection is restored.
A star type light subscriber transmission device using a star coupler is disclosed in Japanese Patent Laid-Open No. 05-153053 (reference 1). According to reference 1, a fault detection circuit and fault detection signal generation circuit are connected to the terminal on an N branching side of the star coupler having a branching of 2: N via an optical directional coupler. A first-station-side light subscriber transmission device is connected to one terminal of the branching side of the star coupler, and a fault detection signal extraction circuit and a second-station-side light subscriber transmission device are connected to the other terminal via an optical branching device. The first- and second-station-side light subscriber transmission devices are switched and controlled by a selection circuit which received the output signal from the fault detection signal extraction circuit.
A dual changeover system using a star coupler is disclosed in Japanese Patent Laid-Open No. 10-294753 (reference 2). According to reference 2, a phase difference calculation means calculates a reception phase difference between an active-system transmission/reception section and a standby-system transmission/reception section of a subscriber-side device while the reception states of the active-system transmission/reception section and standby-system transmission/reception section are normal. A pointer control means then calculates a standby-system transmission phase by using the calculated reception phase difference and the active-system transmission phase.
In references 1 and 2, there is no description about switching of only a virtual path having undergone a fault to the standby system, and there is provided no solution to the above problem associated with switching of normal virtual paths.